Approximately 50% of the energy demand in the United States is currently satisfied by the combustion of coal, and the total energy value of remaining U.S. coal deposits exceeds the energy content of natural reserves for all other types of fuels combined. Retrofitting the national coal infrastructure to use other types of fuels is expected to require many years and enormous investments of new working capital. Indeed, a complete conversion to non-coal energy sources in the near future would drive energy prices to levels that most consumers are not prepared to pay. Thus although the environmental impact of coal use is a source of ongoing controversy, coal will nevertheless remain an important energy resource for the foreseeable future.
The problems with coal use arise partly from its composition and location. Coal, like peat, is a complex biomass material resulting from anaerobic bacterial degradation of accumulated dead plant matter under pressure. Coal is largely carbonaceous, but because of its botanical source coal often contains heteroatoms such as sulfur and nitrogen that undergo chemical processes to form pollutants when burned. The greatest coal reserves are, in fact, the low-ranked varieties having substantial amounts of sulfur and nitrogen. Furthermore the burning of coal converts ambient atmospheric dinitrogen to gaseous brown nitrogen oxides, which are pollutants and characteristic components of smog. And coal combustion also generates large quantities of carbon dioxide, a greenhouse gas that has a role in climate change. Underground coal acts as a type of filter for toxic elements in groundwater, thus coal also contains significant amounts of arsenic, chromium, mercury and radioactive species such as radium.
Direct combustion of coal can also be quite inefficient. This is partly because coal is a very wet material. By weight, coal as mined often contains 30% water or more. When coal is burned in that condition the water is baked off as water vapor in an unrecoverable energy loss unless steps are taken to dry the coal before burning or to harness the energy of the emitted water vapor. Yet even for dry coal the combustion rate is often lower and less uniform than desired.
Coal is also inefficient as a candidate for transport. Traditionally coal has been shipped in lump form by barge or railroad car for long distances. In modern times slurries have been made consisting of approximately 30% finely ground coal by weight in water or light oil; such compositions are suitable for transport by pipeline, and conveniently they are also suitable for burning and can be substitutes for diesel fuel. However slurry generation for a pipeline requires an ample supply of water or oil at the pipeline's outermost point. And conventional coals have many deleterious effects, including eroding and agglomeration, as well as plugging pumps, valves and boiler tubes. For many major coal deposits such as those found in the Midwest, that prerequisite often cannot be satisfied.
Conversion of coal to other fuel sources is also energy-intensive. Its gasification requires temperatures above about 1100° F. at atmospheric pressure: most gasifiers currently operate at 2600-2800° F. because lower temperatures produce large quantities of tars, phenols and coke. Direct liquefaction of coal is typically conducted in the range of 650-750° F. and 20 bars of pressure. Catalysts are often used at up to 2 weight percent relative to coal to aid these processes, however such catalysts are not distributed very finely or uniformly in coal mixtures and thus the catalysis efficiencies are hardly optimal.
Traditional milling of coal also entails high maintenance expense, unpredictable downtime and wide, variable particle size distribution because mechanical crushing or grinding causes considerable wear on exposed parts of the mill.
There is therefore a need for improved coal types, improved combustion processes and related equipment that will result in relatively less pollution, higher efficiency in converting coal to usable energy, higher efficiency in conversion to other fuels, and higher efficiency in milling.